In recent years, attention has been focused on MIMO (Multiple-Input Multiple-Output) as a system effectively using a limited frequency band in a radio communication art for realizing high-speed transmission. The MIMO is an art of transmitting data using multiple antennas in both transmission and reception. Different data is transmitted from multiple transmission antennas, whereby the transmission capacity can be enhanced without enlarging the time and frequency resources.
Applying a multiple codeword (MCW: Multiple codeword) to a data transmission method in the MIMO is effective. The multiple codeword makes it possible to give CRC (Cyclic Redundancy Check) to each data piece of each transmission path formed by multiple transmission antennas and beams and then give an error correction code of convolutional code, Turbo code, LDPC code, etc., and set a modulation system and a coding ratio responsive to SINR (Signal-to-Interference plus Noise power Ratio) corresponding to the transmission path (transmission antenna and beam). In addition to the ability to set such a flexible modulation system and a coding ratio, in the multiple codeword, it is possible to make an error correction based on CRC for each transmission path, so that units of retransmission control can be set to transmission path units (transmission antenna and beam units) and a gain can be obtained efficiently according to the retransmission control.
FIG. 11 is a drawing to schematically show an increase in overhead in HARQ control using a multiple codeword. Here, the case where a signal is transmitted from a wireless base station (BS: Base Station) which becomes a transmission apparatus (transmission station) to a user terminal (UE: User Equipment) of a mobile station which becomes a reception apparatus (reception station) in a cellular system for mobile communications of a mobile telephone, etc., is shown as an example. FIG. 11(A) shows an example of a control signal transmitted in SISO (Single-Input Single-Output) and FIG. 11(B) shows an example of a control signal transmitted in MIMO.
To perform retransmission control based on hybrid-ARQ (Hybrid-Automatic Repeat reQuest) (hereinafter, described as HARQ), in the SISO system, one codeword is used and a pilot signal in a control channel (control CH) and a control signal containing one RV code are transmitted from the transmission apparatus to the reception apparatus. As a response signal, ACK (Acknowledgement) when the reception signals can be normally acquired or NACK (Negative Acknowledgement) when an error is contained in the reception signals is transmitted from the reception apparatus to the transmission apparatus. In contrast, in the MIMO system, to perform HARQ control using a multiple codeword, a pilot signal in a control channel (control CH) and a control signal containing a plurality of RV(redundancy version) parameters provided for each codeword (for each transmission path) are transmitted from the transmission apparatus to the reception apparatus. As a response signal, ACK or NACK for each codeword (for each transmission path) is transmitted from the reception apparatus to the transmission apparatus.
In the HARQ control using the multiple codeword as described above, information of the HARQ control becomes necessary for each codeword. Thus, if the number of codewords become N times, ACK/NACK feedback in the up direction from the reception apparatus to the transmission apparatus required for the HARQ control and signaling of each RV parameter in the down direction from the transmission apparatus to the reception apparatus become each N times. Thus, it becomes necessary to allocate more resource to signaling for transmitting a control signal and a response signal, thus resulting in a decrease in throughput.    Non-patent document 1: 3GPP TSG RAN WG1 #42, R1-050912, Qualcomm Europe, “MIMO proposal for MIMO-WCDMA evaluation”, Aug. 29-Sep. 2, 2005